Method for leaching gold and/or silver out of ores or out of ore-concentrates and also out of precious-metal wastes or precious-metal scrap by using cyanide-containing leaching solutions

ABSTRACT

A method for leaching gold and/or silver out or other precious metal containing substance using cyanide-containing leaching solutions and adding an aqueous H 2  O 2  solution to accelerate leaching. The total amount of the hydrogen-peroxide solution required for leaching is fed into the leaching slurry at the beginning of leaching. The amount of the H 2  O 2  solution is determined in that the H 2  O 2  is added in a 0.6 to 2.5-fold stoichiometric ratio to the cyanide concentration of the leaching slurry. The pH value of the leaching slurry is set and maintained at between 7 and 10 or between 11 and 13.

INTRODUCTION AND BACKGROUND

The invention relates to a method for recovering precious metalcomponents by leaching gold and/or silver out of ores orore-concentrates and also out of precious-metal wastes or precious-metalscrap. Method for recovery of the precious metal values employscyanide-containing leaching solutions having a pH value between 7 and 13by adding an aqueous H₂ O₂ solution.

In the heretofore conventional industrial procedure for leachingprecious metals out of ores or ore-concentrates or other precious metalcontaining substances, the starting material is finely ground anddispersed in water to form a slurry. The pH value of the ore slurry isthen raised into the alkaline pH range by adding lime or milk of lime,whereupon the complexforming cyanide is added, as a rule in the form ofNaCN in an aqueous solution of approximately 5 to 20%. Next the oreslurry is agitated, with introduction of air, for 24 to 48 hours instirring tanks or Pachuca tanks for a time period of 24 to 48 hours.

Because cyanides hydrolyze in the acid pH range while forming the highlytoxic hydrocyanic acid, the pH value of the ore slurry must be raisedinto the alkaline range and be kept there. Ordinarily, the process takesplace in the pH range from 10 to 11. Typically, this pH range is set byadding lime and milk of lime in the ratio of 80:20.

The cyanide concentration of the ore slurry depending on the orecomposition ranges from 0.025 to 0.25 % by weight of NaCN. The cyanideis metered in the form of an aqueous solution of 2 to 20 % into thefirst of up to ten cascaded leaching tanks traversed by the slurry.

With regard to the introduction of air into the leaching slurry, aconcentration of dissolved oxygen in the slurry is chosen to be as closeas possible to the physically determined maximum value of about 8 to 9ppm because at the practicable conventional cyanide concentration of 0.3to 3 kg/ton of ore, the diffusion of the dissolved oxygen in the slurrydetermines the rate of leaching.

The leaching of gold and silver in cyanide solutions when using NaCNtakes place according the formula below: ##STR1##

The above reaction takes place in two steps: ##STR2##

The peroxide formed in the first reaction step because of itsdecomposition into oxygen and water increases the concentration of thedissolved oxygen in the slurry. In lieu of the NaCN shown herein, KCNalso may be used just as well.

It is known to use hydrogen peroxide when recovering precious metals bymeans of cyanide-containing leaching solutions. One process does not usethe hydrogen peroxide to oxidize the precious metals, instead it uses itto detoxify the leaching slurries before they are drained away. Thecyanide present in the slurries is decomposed therein by addition ofhydrogen peroxide into non-toxic cyanate and ammonia. The decompositionof the cyanide takes place relatively rapidly only in the pH range ofthe known leaching procedure initially cited, wherein the oxygen isinserted into the leaching slurry in the form of blown air.

Another known procedure (US patent 3,826,723) provides for adding anaqueous H₂ O₂ solution to preciousmetal leaches by means ofcyanide-containing leaching solutions. This procedure, however, is noteconomical on account of its unusually high consumption of cyanide andwould not be implemented on a commercial scale. In this known procedure,a shorter leaching time requires the cyanide concentration to be between1 to 600 g of NaCN per liter of leach. In an illustrative examplethereof 60 g of NaCN per liter of leach and 5 ml of stabilized 50 % byweight H_(hd) 2 O₂ are added per liter of leach. Moreover the knownprocedure requires adding 1 g of lignine sulfate per liter of leachingsolution.

The molar ratio of NaCN/H₂ O₂ set in the above known procedure indicatesthat operation takes place at a pH value between 9.5 and 11.5 as in thepreviously described detoxification of the leaching slurries. Because ofthe marked decomposition of the NaCN in its reaction with the added H₂O₂ only that NaCN added in excess is available for complexing theprecious metals.

German patent 36 37 082, describes a procedure for the leaching of goldand/or silver out of ores or oreconcentrates employing an aqueouscyanide leaching solution with a pH of 8 to 13 and adding an aqueoussolution of H₂ O₂. However, this prior process is carried out bycontrolling the addition of said H₂ O₂ solution by means of theconcentration of the oxygen dissolved in the leaching solution, theoxygen content in said solution being set at 2 to 20 mg/liter. Thepurpose thereof is to carry out the leaching more 0 economically, inparticular by reducing the H₂ O₂ consumption, without thereby entailinghigher sodium cyanide consumption relative to the previously knownleaching with air blowing.

SUMMARY OF THE INVENTION

The object of the invention is to provide an improved leaching processand to appreciably shorten the leaching time relative to the knownprocedure while operating at the conventional NaCN concentration in theleaching slurry and with low consumption of H₂ O₂, whereby also thecosts of carrying out the method shall be significantly lowered.

In carrying out the present invention, the following features are noted:

(a) the required total amount of the hydrogenperoxide solution requiredfor leaching is added to the leaching slurry at the beginning ofleaching,

(b) the hydrogen peroxide is added in a stoichiometric ratio of 0.6 to2.5 to the cyanide concentration of the leaching slurry, and

(c) the pH value of the leaching slurry is set and kept at between 7 and10 or between 11 and 13.

The invention is based on the discovery that the reaction between NaCNand H₂ O₂ is strongly inhibited, that is it takes place more slowly inthe alkaline pH range below 10 or above 11 and up to 13. As a result,the NaCN is maintained as a complexing agent while simultaneously thedesired concentration of dissolved oxygen in the slurry is obtained bythe decomposition of the H₂ O₂ as required for the reaction between theNaCN and the precious metal as explained initially above. By adding thetotal amount of the H₂ O₂ solution required for leaching to the leachingslurry at the beginning of leaching, an optimal ratio of H₂ O₂concentration to the amount of precious metal to be leached and to theNaCN concentration is achieved over the entire leaching time. At thebeginning of the leaching process, the greatest amounts of H₂ O₂ will beoffered when the leaching slurry contains the highest amounts ofprecious metals to be leached and the highest amounts of cyanide. As theprecious metal is increasingly bound to the CN, the contents in H₂ O₂ inthe leaching slurry automatically adjust themselves to the lesseneddemand.

In order to accelerate the leaching process, it is necessary that assoon as the H₂ O₂ solution is introduced, it be homogeneously dispersedin the leaching slurry because otherwise local detrimental molar ratiosNaCN/H₂ O₂ will occur in this leaching slurry which would result inaccelerated decomposition of the NaCN whereby either the output ofprecious metals would be lowered or the necessary addition of NaCN wouldbe increased. Accordingly, it is advantageous for the hydrogen-peroxidesolution to be added continuously and homogeneously dispersed across thecross-section of a flow of leaching slurry. Because of the homogeneousdispersion of the H₂ O₂ solution in a leaching slurry flow, a uniformreaction across the flow cross-section is moreover achieved from thetime of addition, whereby again the leaching time is shortened.Especially advantageous ratios and leaching times are obtained when thepH of the leaching slurry is set between 8 and 9.5 or between 11.5 and12.3.

The H₂ O₂ solution can be added to the leaching slurry both before orafter the cyanide is added. Advantageously, the addition of the H₂ O₂solution is added before the cyanide is introduced into the leachingslurry because thereby an O₂ enrichment and dispersion in the slurry canbe achieved before the cyanide addition.

Just as in the conventional procedure, the NaCN concentration is about0.25 to 1 g per liter of the leaching slurry. It was ascertained by aseries of tests that when liter of leaching slurry at a pH of 9, almost100% of the precious metals can be extracted from this slurry afterabout 100 minutes. The precious metals can be separated in problem-freemanner by charcoal adsorption.

The new method reduces the time for leaching precious metals to 1/5 to1/20 of the times needed by the conventional procedures of the state ofthe art. Instead of the previous leaching times of 24 to 48 hours timesof 2 to 4 hours are sufficient when using the new method.

The amount of the added H₂ O₂ solution is not limited by the quantity ofNaCN present in the leaching slurry. Rather the H₂ O₂ amount may beclearly higher than that required for complete decomposition of theNaCN, without having to incur an undesired decomposition of the NaCN.Practice has shown it is advantageous to add the H₂ O₂ solution in sucha quantity that the amount of the H₂ O₂ is 0.1 to 2.5 kg per ton of ore,for typical cyanide concentrations of 0.1 to 3 kg of NaCN per ton ofore.

An advantageous mode of introduction of the H₂ O₂ solution into the flowof leaching slurry is carried out by feeding the H₂ O₂ solutioncrosswise to this flow of leaching slurry at 10 to 50 times the flowrate of the flow of leaching slurry. A preferred mode of implementationis characterized in that the leaching slurry is guided at a rate of 1 to3 meters/sec. in the feed region of the H₂ O₂ solution and, further, inthat the cross-flow of the H₂ O₂ solution is set for a rate of 10 to 100meters/sec.

The feed of the H₂ O₂ solution into the flow of leaching slurry iscarried out by conventional feed devices, preferably by slit-nozzlesdimensioned in the micron range. By means of the above steps, uniformdispersion of the H₂ O₂ into the flow of the leaching slurry is achievedwithin fractions of a second. When using slit-nozzles, slit apertures of5 to 100 microns are recommended if the H₂ O₂ solution is added to theleaching slurry in the area of tubular cross-section having a diameterno larger than 10 cm.

If high proportions of cyanide-soluble iron or copper, in particular inthe form of sulfides, are present in the precious-metal ores orore-concentrates, then because of the catalytic effect of the Cu(II ofFe(III) ions or other equally acting metal ions, spontaneousdecomposition of the H₂ O₂ in the leaching slurry may take place andhence also rapid decomposition of the cyanide. These reactions occurthroughout the entire pH range from 7 to 13, even in the above citedrange between 7 to 10 or 11 to 13.

The above drawbacks can be avoided by adding flotation reagents oranionic polymers in amounts of 5 to 1,000 g per ton of ore, preferably25 to 250 g per ton, to the leaching slurry, in order to passivate orsequester the copper or iron minerals. Preferably the addition of theflotation reagents or polymers takes place after the pH has been set inthe leaching slurry, or before the H₂ O₂ solution and the cyanide areadded. Preferred sequestering or passivating agents are thiocarbonates,thiophosphates, thiocarbamates or anionic polymers in the form ofacrylates, starches, and carboxymethyl cellulose. The passivating orsequestering agents as a rule are added in the form of a 0.1 to 10 % byweight aqueous solution to the leaching slurry. Any suitable agent ofthis type can be used for purposes of the invention.

The addition of the reagents as described herein advantageously shouldalso take place in homogeneously dispersed manner over the cross-sectionof the slurry flow. When using aqueous solutions, this can be carriedout using slit-nozzles, as already discussed in relation to the feed ofthe H₂ O₂.solution The homogeneous or short-term dispersion of thepassivating of sequestering agents results in shortening their reactiontimes with Cu or Fe minerals respectively.

DETAILED EMBODIMENTS OF INVENTION Examples

The following results were obtained for a 2 liter glass beaker in theexperiments carried out:

(A) Leaching conventionally with NaCN without adding H₂ O₂.

A gold ore rich in malachite/azurite (this is the leaching residue of ashut down gold-processing plant) contains about 3 ppm gold and 1.2 %copper as an important accompanying element. The gold ore is ground to ad₈₀ value of 200 microns.

The ore is leached at a slurry density of 500 g of ore per liter ofleach while being agitated with turbulence and with addition of NaCNbetween 0.02 to 0.2 % by weight. The pH values of the leaching slurriesare varied between 9 and 12.

The gold yields in no case exceed 87 % in these tests. Maximumextraction in each case is achieved only after an approximate leachingtime of 20 hours.

(B) Leaching the same ore as in (A) but with H₂ O₂ addition.

0.6 ml of 35% H₂ O₂ solution per liter of leach is fed throughslit-nozzles, before the introduction of the NaCN, to the ore slurryhaving a density of 500 g of ore per liter of leach. This is carried outin such a way that a NaCN concentration of the leach of 0.05 % by weightis obtained. For the described addition of the H₂ O₂ solution, the H₂ O₂concentration of the leach is 0.02 % by weight.

After a test of 3 hours and for the leaching slurry adjusted to 8 and 9pH extractions of gold in excess of 97 % are achieved. When the pHvalues of the leaching slurry are 12, the extractions also are higherthan 90%, namely 93 %. A gold yield less than 87 % is observed only inthe range between the said pH values.

(C) Leaching a partially refractory pyrite-containing without adding H₂O₂ (gold content =1.3 ppm).

The ore ground to a d₈₀ value of 200 microns is reacted at a slurrydensity of 500 g of ore per liter of leach with 0.7 kg NaCN per ton ofore and is aerated by air being blown in. The slurry pH was set at 11.

After 24 hours a maximum gold extraction is achieved at about 62 %.After 3 hours, the gold extraction is 45%.

(D) Leaching the ore in (C) with addition of H₂ O₂.

The slurry of (C) with the same amount of NaCN and after its pH valuehas been set to 9 receives the peroxide solution until the amount of H₂O₂ per ton of ore is 1.6 kg.

After leaching for 3 hours a gold extraction of 91 % is obtained.

(E) Leaching a gold-ore from Southeast Asia containing copper and ironsulfide (gold content=6 ppm).

Control tests were carried out when leaching the ore by adding an H₂ O₂solution with and without additives to passivate/sequester thecopper/iron sulfide minerals; the results are shown in the Table below.

    ______________________________________                                                                   pH 9   pH 9  pH 9                                           pH 2.88   pH 9    H.sub.2 O.sub.2                                                                      H.sub.2 O.sub.2                                                                     H.sub.2 O.sub.2                       Au extraction                                                                          air       H.sub.2 O.sub.2                                                                       AX     CMC   TTC                                   ______________________________________                                        after 3 h.                                                                             25%       55%     90%    92%   >97%                                  after 24 h.                                                                            68%       58%     91%    93%   >97%                                  ______________________________________                                    

In all tests the slurry density was 500 g of ore per liter of leach andthe addition of NaCN was 0.8 kg per ton of ore. The H₂ O₂ solution wasadded until 1 kg of H₂ O₂ was present per ton of ore.

In the Table, the abbreviations are as follows: AX=amylxanthogenate,TTC=trithiocarbonate, CMC=carboxymethylcellulose. These aforementionedpassivating/sequestering agents were added at the rate of 0.1 kg per tonof ore.

The Table shows that absent passivating sequestering the copper and ironsulfide, only a gold extraction of a maximum of 68 % is possible at aslurry pH of 11 and only of 55 % at a slurry pH of 9, and that furtheraddition of H₂ O₂ takes place, whereas an increase in extraction isachieved already after leaching for 3 h up to 97% when adding the saidpassivating sequestering reagents.

Further variations and modifications will be apparent from the foregoingto those skilled in the art and are intended to be encompassed by theappended claims.

We claim:
 1. A method for leaching precious metals selected from gold,silver and mixture thereof out of a precious metal containing substanceselected from ores, ore concentrates, precious metal wastes and preciousmetal scrap comprising contacting an aqueous leaching slurry of the saidsubstance with a cyanide-containing leaching solution and an aqueoushydrogen-peroxide solution, wherein:(a) the total amount of thehydrogen-peroxide solution required for leaching is added to saidleaching slurry at the beginning of leaching, and before the cyanide isadded, wherein said leaching slurry is in a flow and thehydrogen-peroxide solution is introduced as a cross-flow into the flowof the leaching slurry at a flow-rate which is 10 to 50 times higherthan the flow-rate of the leaching slurry, and the peroxide solution ishomogeneously dispersed in the cross-section of said flow, (b) thehydrogen peroxide is added in a 0.6 to 2.5-fold stoichiometric ratio tothe cyanide concentration, wherein the H₂ O₂ solution is added in suchan amount that the proportion of the H₂ O₂ is between 0.1 and 2.5 kg perton of said substance for cyanide concentrations between 0.1 and 3.0 kgof NaCN per ton of said substance, and (c) the pH value of the leachingslurry is set and maintained at alkaline pH selected from pH valuebetween 8 and 9.5 and pH value between 11.5 and 12.3.
 2. The methoddefined in claim 1, wherein the leaching slurry is guided in the areawhere the H₂ O₂ solution is fed-in at a flow-rate of 1 to 3 m/s and inthat the cross-flow of the H₂ O₂ solution is set to a flow-rate of 10 to100 m/s.
 3. The method defined in claim 1, wherein the H₂ O₂ solution isfed through slit-nozzles dimensioned in the micron range into the flowof the leaching slurry.
 4. A method for leaching precious metalsselected from gold, silver and mixtures thereof out of a precious metalcontaining substance, wherein said substance is selected from ores andoreconcentrates in which cyanide-soluble non-precious metals selectedfrom iron, copper and mixtures of same are present, comprisingcontacting an aqueous leaching slurry of the said substance with acyanide-containing leaching solution and an aqueous hydrogen-peroxidesolution, wherein:(a) the total amount of the hydrogen-peroxide solutionrequired for leaching is added to said leaching slurry at the beginningof leaching, and before the cyanide is added, wherein said leachingslurry is in a flow and the hydrogen-peroxide solution is introduced asa cross-flow into the flow of the leaching slurry at a flow-rate whichis 10 to 50 times higher than the flow-rate of the leaching slurry, andthe peroxide solution is homogeneously dispersed in the cross-section ofsaid flow, (b) the hydrogen peroxide is added in a 0.6 to 2.5-foldstoichiometric ratio to the cyanide concentration, wherein the H₂ O₂solution is added in such an amount that the proportion of the H₂ O₂ isbetween 0.1 and 2.5 kg per ton of said substance for cyanideconcentrations between 0.1 and 3.0 kg of NaCN per ton of said substance,(c) the pH value of the leaching slurry is set and maintained atalkaline pH selected from pH value between 8 and 9.5 and pH valuebetween 11.5 and 12.3, before contacting the slurry with the hydrogenperoxide solution and (d) further comprising the addition ofpassivating/sequestering agents selected from the group consisting ofthiocarbonates, thiophosphates, thiocarbamates, acrylate anionicpolymers, starch anionic polymers and carboxymethyl cellulose anionicpolymers in a proportion of 25 to 250 g per ton of ore, to the leachingslurry, in order to passivate/sequester the said non-precious metals,wherein the addition of said agents is made after the pH-setting of theleaching slurry and before the H₂ O₂ solution and the cyanide are added,and wherein the addition of passivating/sequestering agents is fed inthe form of a 0.1 to 10% by weight aqueous solution to a flow ofleaching slurry in cross-sectionally homogeneously dispersed manner.